1. Field of the Invention
This invention concerns a crystal support device to support nonlinear optical crystals that emit ultraviolet light. Specifically, it concerns a crystal support device that prevents beam fluctuations that arise when the crystal temperature is higher than room temperature.
2. Description of Related Art
In recent years there have been attempts to generate ultraviolet light through wavelength conversion using nonlinear optical crystals. A variety of crystals are known to be nonlinear crystals that can be used for such applications, including CLBO (CaLiB6O10), LBO (LiB3O5), BBO (xcex2-BaB2O4), CBO and (CaB3O5).
Many of these nonlinear crystals are used by elevating the crystal temperature above the ambient temperature; these include LBO, CLBO, CBO, PPLN, KTP, LN crystals, and also KDP and KDP*P.
In applications run at an elevated crystal temperature, the air at the optical beam""s entrance to and exit from the crystal support device comes in direct contact with the high-temperature crystal support device, which causes fluctuations in the density of the air and produces instability due to the continual changes in the index of refraction at the beam entrance and exit of the crystal support device.
As shown in FIG. 10(a), when the temperature of the beam entrance 10a and exit 10b of the crystal support device 10 is higher than the ambient temperature, fluctuations occur in the air nearby. Because the index of refraction of air varies with its temperature, regions of differing temperature are formed near the beam entrance 10a and exit 10b, and if domains in these regions fluctuate, there will be fluctuations in the laser beams entering and exiting the crystal support device 10. For example, if there is a 10xc2x0 C. difference of temperature between region A and region B shown in FIG. 10b, it will produce a change in the index of refraction of air on the order of 10xe2x88x925 at ultraviolet wavelengths. For that reason, the angle of incidence of a beam that enters region B from region A of FIG. 10b will be xcex81 and the angle of departure to region B will be xcex82. Taking the index of refraction in region A as n1 and the index of refraction in region B as n2, it comes to xcex82/xcex81=n1/n2=10xe2x88x926. As an approximate calculation, the change of beam position arising from the difference in index of refraction will be about 10 xcexcrad.
As stated above, beam fluctuation arises from what is called the air shimmer phenomenon in crystal support devices that are used by elevating the crystal temperature above room temperature. This change of beam position has not been a problem in equipment using lasers heretofore. In recent years, however, the increased power of ultraviolet lasers has been accompanied by the use of ultraviolet lasers for opening holes in precision-printed substrates for high-density mounting. This application requires processing to open holes with diameters of 10 xcexcm, which means the precision of positioning is a few microns. The beam pointing stability required of the laser, converted to angular shift, is a few xcexcrad.
Within the present market for wavelength conversion equipment and other laser equipment, there is much wavelength conversion equipment that is maintained at high temperatures, and it has not been possible to achieve the beam pointing stability required by this market.
In view of the foregoing, a primary object of the present invention is to reduce beam fluctuation between the entry and exit ends of crystal support devices that are used by elevating the crystal temperature above room temperature, and to respond to new market demands.
This object, to prevent beam fluctuation produced when a nonlinear optical crystal is maintained at a high temperature, is achieved in accordance with present invention which establishes a beam passage component at the beam entrance and exit sides of the nonlinear optical crystal beam of the crystal support device, and attempts to control the temperature at the end of the beam passage component away from the crystal within a range of difference from ambient temperature at which the beam will be stable.
A hollow passage component that will not produce a counterflow of air inside it, or a solid passage component of a material like quartz with high transparency to the wavelength of laser beams can be used as the beam passage component.
It is even more effective if the end of the beam passage component away from the crystal is cooled to a temperature close to the ambient temperature.
It was learned from experimentation that the beam fluctuation could be kept within a given value S (in xcexcrad) by controlling the temperature difference T between the temperature of the beam passage component away from the crystal on the entering and exiting sides and the ambient temperature such that Txe2x89xa620+3.5S.
A focusing lens is generally placed between the exit of the laser beam and the item to be treated by laser beam, and the amount of fluctuation of the beam on the surface of the item to be treated is a fraction of the amount of fluctuation at the exit of the laser beam exit. Accordingly, if there is a distance of 1 m between the exit of the laser beam and the item to be treated, when the amount of beam fluctuation at the exit of the laser beam exit is 5 xcexcrad, there would be a 5 xcexcm fluctuation on the surface of the item to be treated. However, since the focusing lens will reduce the amount of fluctuation on the surface to a fraction, the amount of fluctuation on the surface of the item to be treated may be 1 xcexcm or less, and it will be possible to maintain the necessary precision of processing.
Now, it became clear through experimentation that the beam passage component would have to be set up on both the beam-entrance and beam-exit sides of the crystal. That is, if an optical item were placed just on the beam-exit side, the beam would fluctuate on the beam-entrance side, and so the beam pointing stability would deteriorate from that when the optical item were placed on both the beam-entrance and beam-exit sides.
If the beam passage component is a hollow tube, then of course the smaller its cross-sectional area, the more effectively the movement of air within the tube can be limited. Experimentation confirmed, however, that the effect was greatest if the maximum diameter of the hollow tube were no more than 3 times the length of the diagonal of the crystal. The reason for that is thought to be as follows. There is naturally convection in the flow of air within the hollow tube. Because one end of each hollow tube is connected to the crystal holder, air cannot flow in from the other end. Consequently, if the temperature at the opposite, open end is slightly lower, the speed of air flow within the hollow tube is extremely low. Accordingly, fluctuation of the index of refraction can be almost eliminated, and there is almost no positional shift of the beam passing through the hollow tube.
If the minimum value for the inside diameter of the beam passage component is 2 times the beam diameter, it can be assured that there will be no obstruction to passage of the laser beam. As used here, the beam diameter is the width p/e2 (where e is the base of the natural logarithm: 2.71828), when the peak value of the energy distribution is p; here is, the energy distribution of the laser beam is a Gaussian distribution with M2=1 (where M2 is the parameter for evaluating the quality of the beam).
Now, if there is, around the gap between the end of the beam passage component that is near the crystal and the face of the crystal, a cover that covers the full circumference, it is possible to prevent the occurrence of beam fluctuation caused by convection within that gap. If there is a vacuum around the optical crystal and the space enclosed by the beam passage components, it is possible to reliably prevent beam fluctuation within the path comprising the optical crystal, the beam passage components and the hollow tube.